This invention relates to a printing system, and more particularly, to a line printing system which is capable of simultaneously transferring all pixel information of one raster line or one text line through use of an array of light valves.
It has been shown that an electro-optic element having a plurality of individually addressable electrodes can be used as a multi-gate light valve for line printing. See, for example, U.S. Pat. No. 4,281,904 on a "TIR Electro-Optic Modulator with Individually Addressed Electrodes" and U.S. Pat. No. 4,389,659 on an "Electro-Optic Line Printer". Also, see "Light Gates Give Data Recorder improved Hard Copy Resolution," Electronic Design, Jul. 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Waveforms," Machine design, Vol. 51, No. 17, Jul. 26, 1779, p. 62; and "Data Recorder Eliminates Problem of Linearity," Design News, Feb. 4, 1980, pp. 56-57.
Almost any optically transparent electro-optic material can be used as the electro-optic element of a light valve such as LiNbO.sub.3, BSN, KDP, KD.sub.x P, Ba.sub.2 NaNb.sub.5 O.sub.15 and PLZT. The electrodes of such a light valve are coupled to the electro-optic element and are distributed in non-overlapping relationship widthwise of the electro-optic element (i.e. orthogonally relative to its optical axis), typically on equidistantly separated centers so that there is a generally uniform inter-electrode gap spacing.
To perform line printing with a multi-gate light valve of the foregoing type, a photosensitive recording medium, such as a xerographic photoreceptor, is exposed in an image configuration as it advances in a cross line direction relative to the light valve. More particularly, to carry out the exposure process, a sheet -like collimated light beam is transmitted through the electro-optic element of the light valve, either along its optical axis for straight through transmission or at a slight angle relative to that axis for total internal reflection. Furthermore, successive sets of pixel information are sequentially applied to the electrodes.
In the case of straight through transmission, a light valve is positioned between two polarizing filters, the axis of one filter oriented at 90 degrees with respect to the other to totally block the passage of light. An unpolarized collimated light beam will be polarized linearly by the first filter, along the horizontal axis. When the gates are not activated, the polarized light is blocked by the second filter. However, the activated light gate rotates the plane of polarization of light 90 degrees so that it passes through the second filter. Light, which is passed through, is then focused by a lens as a spot on a photoreceptor.
In the case of total internal reflection, an electro-optic element (a crystal with multi-surface) is arranged such that a collimated beam of light of single wavelength incident at an angle to the plane of one surface is refracted at the other surfaces to incur total internal reflection at the entrance surface. By applying a voltage to the electro-optic element which induces an electric field adjacent to the light entrance surface, the refractive index of the electro-optic material will change, thereby frustrating the total internal reflection. The reflected light at a certain angle will then be focused on a photoreceptor.
The above printing systems are not efficient users of light source. Furthermore, the multi-gate light valve in each case requires a significant voltage, typically between 10 and 20 volts, to switch each pixel and therefore is not directly compatible with low-voltage silicon circuits. In addition, assembly of such multi-gate light valves is cumbersome and unreproducible, thereby having a low yield and high price. Finally light valves of this type utilize electro-optic materials which are not monolithically compatible with semiconductors so that integration of driver electronics with the modulator is not possible.